1. Field of the Invention
The present invention relates to a power supply apparatus, and more particularly, to a power supply apparatus capable of selecting and supplying power from one of a plurality of inputs.
2. Related Art
An electric apparatus, such as a mobile printer, a notebook personal computer (PC) or other mobile device, may include a power supply apparatus comprising an adapter to convert an external power and a battery provided inside thereof, and a device to properly select or control between the power provided from the adapter (hereinafter, referred to as “adapter power”) and the power provided from the battery (hereinafter, referred to as “battery power”) based on a current state of the electric apparatus.
FIG. 1 illustrates a typical electric apparatus. As shown in FIG. 1, such an electric apparatus 10 comprises a power selecting unit 11, a power switching unit 12 and a system unit 13. The system unit 13 comprises actual components of every configuration, except for a configuration connected to power, for example, a printing engine (not shown), a user interface (not shown) and so on. The power selecting unit 11 selects between the adapter power Va and the battery power Vb according to the condition, and supplies the selected power to the power switching unit 12. The power switching unit 12 then transmits the selected power Vx from the power selecting unit 11 to the system unit 13 according to a system power on/off signal (hereinafter, referred to as “power switching signal”) which is inputted by a user. Herein, the power selecting unit 11 and the power switching unit 12 can be referred to as a power supplying apparatus.
In the conventional power supplying apparatus, the power switching unit 12 is provided at an output terminal of the power selecting unit 11 to directly control a system power Vin which is the main power. In addition, multiple high-cost field-effect transistors (FETs) and a separated control device are required for a high-power application. As a result, the conventional power supplying apparatus can be large and expensive.
Specifically, FETs are incorporated into the power selecting unit 11 as well as the power switching unit 12. In the situation where the power selecting unit 11 and the power switching unit 12 are combined, a plurality of FETs are connected in series so that a value of voltage can be dropped by each circuit element. Therefore, a voltage value of the system power Vin (hereinafter, referred to as “input voltage level”) may not be in the range which is needed in the system unit 13 because of the voltage drop. However, if the FETs having a low resistance value are used for a high-power application to ensure that the input voltage level is at a level needed in the system unit 13, the production cost can be very high. Furthermore, if the input voltage level is at the level needed in the system unit 13, the range of usable voltage in the battery power Vb (for example, generally it is between 3V and 4.2V) becomes more narrow such that the battery power Vb cannot be sufficiently used. As a result, the life of the battery can be shortened.